This invention relates to an antenna tuning circuit in electronic tuning type radio receivers.
In antenna tuning circuits for AM car receivers employing an electronic tuning system, a circuit as shown in FIG. 1 has been in use. In an equivalent circuit shown in FIG. 2, an antenna section ANT has an antenna capacitance C.sub.B of about 15 pF, and a cable capacitance C.sub.D of about 65 pF which, together with a capacitance of about 20-40 pF added in a receiver stage, form a great capacitance of about 100-120 pF. Since not sufficient capacitance varying ratio can be obtained in a tuning circuit consisting of a variable capacitor C.sub.1 and an inductance L wherein a variable capacitance diode (hereinafter referred to as a varactor diode) is used as the variable capacitor in the tuning circuit because of the large capacitance connected in parallel with the inductance L on the antenna side, a circuit constitution as shown in the figures is employed wherein the same varactor diode as the diode C.sub.1 is connected in series to the antenna.
In these figures, reference TR represents a radio frequency transformer having a winding ratio of l: 1 whose secondary coil is connected to a radio frequency amplifier RFA. In FIG. 2, E.sub.1 represents electromotive force induced on the antenna. The variable capacitor C.sub.2 which is the same as the variable capacitor C.sub.1 in the tuning circuit is connected in series to the antenna for the equivalent decrease in the antenna capacity, taking S/N ratio into consideration. As apparent from FIG. 2, the antenna electromotive force E.sub.1 is divided between the antenna capacitance C.sub.B and the cable capacitance C.sub.D and the resulted voltage is further divided between capacitors C.sub.2 and C.sub.1 and applied to the ratio frequency transformer TR. If the capacitor C.sub.2 is a fixed one, the divided voltage changes with the variation in the capacitance of the capacitor C.sub.1 wherein the voltage applied to the radio frequency transformer TR is decreased with the increase in the capacitance of the capacitor C.sub.1 to worsen the S/N ratio. Changes in the capacitance of the capacitor C.sub.2 along with those of the capacitor C.sub.1 make the voltage divisional ratio and thus the S/N ratio constant.
Output voltage E.sub.2 from the above antenna circuit is represented by the following equation and shown by the graph as in FIG. 3: ##EQU1## wherein P=j.omega. and C is the capacitance of the capacitors C.sub.1 and C.sub.2. As can be seen from FIG. 3, while the output voltage E.sub.2 arrives the peak at the resonance angular frequency .omega..sub.o and rapidly decreases thereafter, the voltage decrease is limited to a certain level, E.sub.2s, below which no reduction is resulted no matter how the angular frequency increases. The resonance angular frequency .omega..sub.o and the above certain level E.sub.2s are represented by the equations: ##EQU2##
If the output voltage E.sub.2 is not decreased below a certain level beyond the resonance angular frequency, then the interference ratio will be considerably worsened unless a highly selective device is added to the tuning circuit at the radio frequency amplifying stage or the like.